Usually insignificant.
$2300 Onkyo 3010: 189WPC x 2Ch into 8 ohms @ 1% THD, 147WPC x 5Ch, 127WPC x 7Ch.
$2500 Pioneer SC-68 (also 9Ch): 191WPC x 2Ch, 138WPC x 5Ch, 113WPC x 7Ch.
$1100 Denon 3312: 143.3 WPC x 2Ch, 103.0 WPC x 5Ch, 96.8 WPC x 7Ch.
Doubling the amplifier wattage results in ONLY a 3dB increase in volume ("slightly louder"). Switching from a 50 watt amplifier to a 100 watt amplifier or from a 100 watt amplifier to a 200 watt will only increase the volume by 3 dB as long as you use the same speakers.
As you can see, 127WPC vs 113 WPC vs 96WPC is really insignificant.